Charge coupled device

ABSTRACT

A charge coupled device including a first electrode consisting of a first region and a second region having lower resistance than the first region, and a second electrode consisting of a first region and a second region having lower resistance than this first region. The first region of the first electrode is adjacent to the first region of the second region at an interval of an insulating film. Capable of utilizing the force of electrical field, the device is superior in charge transfer efficiency as well as charge transfer velocity. It also has the capability to improve the performances of high picture quality solid state image sensing devices and time delay devices, which both necessitate a charge coupled device and operate at high frequencies. Additionally, a solid state image sensing device employing this device is not degraded in a dark state by generating a few pulse charges.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to charge coupled devices andmethods for the fabrication of the same. More specifically, the presentinvention is concerned with a structure of charge coupled electrodecapable of improving charge transfer efficiency, and methods forfabricating the same.

A charge coupled device, which transfers pulse charges in one directionby use of the potential difference within a semiconductor device inducedby the potential difference applied to each transfer electrode and whichis widely utilized for a solid state image sensing device, and signaldelay, is generally structured to have an array of fine transferelectrodes which are separated from one another by an insulating film ona silicon substrate.

For better understanding of the background of the present invention, adescription of conventional charge coupled devices is to be given withreference to some drawings.

Referring to FIG. 1, there is shown a conventional structure of a chargecoupled device. Such structure of charge coupled device is fabricated asfollows.

First, into a p type silicon substrate 1 n type impurity ions areimplanted, to form a buried charge coupled device (hereinafter referredto as "BCCD") region 2 over which an oxide film 3 serving as aninsulating film is then formed entirely.

Over the oxide film 3, there is deposited a conductive layer ofpolysilicon, which is subsequently patterned by use of photoetch, so asto form a plurality of spaced-apart, parallel first transfer electrodes13.

Thereafter, using the first transfer electrodes 13 as a mask, a barrier9 is settled on the surface of the BCCD region 2 by ion implantation.,and the first transfer electrodes 13 are insulated by an oxide film.Then, a plurality of spaced-apart, parallel second transfer electrodes14 are formed of polysilicon between the first transfer electrodes.

In such charge coupled device fabricated, each of the first transferelectrodes 13 is paired with a nearby one of the second transferelectrodes 14. Each of these electrode pairs is connected with either afirst clock pulse and a second clock pulse (HΦ₁, HΦ₂) which arealternatively applied to the electrode pairs.

Referring now to FIG. 2, there is illustrated the operating principle ofthe conventional two-phase charge coupled device. While FIG. 2A is anexample of the first and the second clock pulses applied to the transferelectrode of the two-phase charge coupled device, FIG. 2B shows apotential distribution induced within a semiconductor when applying thefirst and the second clock pulses to the transfer electrodes, and amigration course of charges according to the potential distribution.

In detail, at t=1, the first clock pulse (HΦ₁) is in a state of "low",whereas the second clock pulse (HΦ₂) is in a state of "high". At themoment, a potential well becomes deepest at below the transfer electrode13, so that the pulse charges are trapped ill this well.

At t=2, the first clock pulse (HΦ₁) is in a high state, whereas thesecond clock pulse (HΦ₂) is in a low state. Accordingly, the deepestpotential well is formed below the first transfer electrode 13 appliedwith the first clock pulse (HΦ₁), and the potential well of the secondtransfer electrode 14 applied with the second clock pulse (HΦ₂) isrisen, so that pulse charges move into below the first transfer 13 whichhas the deepest well and which is applied with the first clock pulse(HΦ₁).

At t=3, the pulse charges move like at t=2. In this point, the pulsecharge has directivity to move only rightward by virtue of a potentialbarrier formed below the left electrode of the transfer electrode pairconsisting of the first transfer electrode and the second transferelectrode.

Repetition of such first and second clock pulses (HΦ₁, HΦ₂) allows thepulse charge to be transferred.

However, the conventional charge coupled device is problematic in chargetransfer. To scrutinize the potential distribution shown in FIG. 2B, itcould be found that the edge portion below each the transfer electrodesshows a rapid change of the potential which allows the pulse charge tosmoothly move thereat, whereas since the central portion showsequipotential distribution, the pulse charge moves into a neighboringelectrode by not the force of electrical field but only diffusion bywhich the charge transfer is slow in velocity as well as difficult tocomplete.

Such phenomena are more apparent as the frequency applied to thetransfer electrode increases. Therefore, the conventional charge coupleddevice operates in low performance at high frequencies.

SUMMARY OF THE INVENTION

For solving the problems, the present inventors have recognized thatthere exists a need for a novel structure of charge coupled device,superior in transferring charges, and found that the efficiency ofcharge transfer can be improved by delaying the rising time and fallingtime of the clock pulses applied to transfer electrodes in thetransferring direction within the transferring electrode, so as to formpotential gradients in a channel which transfers the charges.

In accordance with an aspect of the present invention, there is provideda charge coupled device comprising: a first electrode consisting of afirst region and a second region having lower resistance than the firstregion; and a second electrode consisting of a first region and a secondregion having lower resistance than this first region, the first regionof the first electrode being adjacent to the first region of the secondregion at an interval of an insulating film.

According to another aspect of the present invention, there is provideda method for the fabrication of a charge coupled device, comprising thesteps of: making a second conductive charge coupled region over a firstconductive semiconductor substrate; coating a first insulating film onthe second conductive charge coupled region; forming a first conductivelayer over the first insulating film; forming a plurality ofspaced-apart, first low resistance regions in predetermined portions ofthe first conductive layer, the first low resistance regions havinglower resistance than the first conductive layer; patterning the firstconductive layer comprising the first low resistance region, so as toform a plurality of first electrodes each of which has one of the lowerresistance regions in one side; coating the entire resulting substratewith a second insulating film; constructing an impurity region in thevicinity of the surface of each charge coupled region between the firstelectrodes forming a second conductive layer on the second insulatingfilm; forming a plurality of spaced-apart, second low resistance regionsin the second conductive layer, each of which has lower resistance thanthe second conductive layer and is adjacent to each of the first lowresistance regions of the first electrode; and patterning the secondconductive layer comprising the second low resistance regions in such away to include the second low resistance region in the side of the firstlow resistance region, so as to form a plurality of second electrodeseach of which exists between the first electrodes and comprises one ofthe second low resistance regions.

According to a further aspect of the present invention, there isprovided a method for the fabrication of charge coupled device,comprising the steps of: making a second conductive charge coupledregion over a first conductive semiconductor substrate; coating a firstinsulating film on the second conductive charge coupled region; forminga first conductive layer over the first insulating film; patterning thefirst conductive layer to form a plurality of spaced-apart, firstelectrode patterns; coating the entire resulting substrate with a secondinsulating film; constructing an impurity region in the vicinity of thesurface of each charge coupled region between the first electrodepatterns; forming a plurality of spaced-apart, second electrode patternson the charge coupled region, each of which is as high as the firstelectrode pattern and present between the first electrode patterns;coating the entire resulting substrate with a third insulating film;forming a low resistance region in both one portion of each the firstelectrode patterns and one portion of each the second electrodepatterns, so as to form at once a first electrode and a secondelectrode, the one portion of each the first electrode patterns beingadjacent to the one portion of each of the first electrode patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and other advantages of the present invention willbecome more apparent by describing in detail the preferred embodiment ofthe present invention with reference to the attached drawings in which:

FIG. 1 is a schematic, cross sectional view illustrating a structure ofa conventional charge coupled device;

FIG. 2A shows an example of first and second clock pulses applied to thetransfer electrode of two-phase charge coupled device;

FIG. 2B is a schematic view illustrating the relation of theconventional structure with a potential distribution induced within asemiconductor when applying the first and the second clock pulses to thetransfer electrodes;

FIG. 3 is a schematic, cross sectional view illustrating a structure ofa charge coupled device according to a first embodiment of the presentinvention;

FIG. 4 is a schematic, cross sectional view illustrating a structure ofa charge coupled device according to a second embodiment of the presentinvention;

FIG. 5A shows an example of first and second clock pulses applied to thetransfer electrode of the charge coupled device according to the presentinvention;

FIG. 5B is a schematic view illustrating the relation of the presentstructure with a potential distribution induced within a semiconductorwhen applying the first and the second clock pulses to the transferelectrodes;

FIGS. 6A through 6G are schematic, cross sectional views illustrating amethod for the fabrication of the charge coupled device according to thefirst embodiment of the present invention; and

FIGS. 7A through 7G are schematic, cross sectional views illustrating amethod for the fabrication of the charge coupled device according to thesecond embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings, wherein likereference numerals designate like parts, respectively.

Referring initially to FIG. 3, there is shown a charge coupled deviceaccording to a first embodiment of the present invention. As shown inthis figure, the charge coupled device is comprised of a firstconductive semiconductor substrate 20, e.g. p type silicon substrate,overlaid by an n type BCCD region 21, a second conductive charge coupledregion, covered with an insulating film 22 on which a plurality of firstregions 23 and a plurality of lower resistant second regions 24 areformed, each of the former along with nearby one of the latterconstituting a first transfer electrode 25 on the edge portion of whicha second transfer electrode 30 consisting of a second region 28 and alow resistant second region is put, extending over the BCCD regionbetween the spaced-apart first transfer electrodes 25, said first region23 of the first transfer electrode 25 being adjacent to the first region28 of the second transfer electrode at an interval of the insulatingfilm 26 and said second region 24 of the former 25 being adjacent to thesecond region 29 of the latter 30 at an interval of that, as well.

The charge coupled device according to the present invention alsocomprises an impurity region 27 to serve as a barrier which is formedbelow each of the second transfer electrodes in the vicinity of thesurface of the BCCD region 21. In the first transfer charge 25, thefirst region 23 is more extensively formed than the second region 24,whereas, in the second transfer charge 30, the second region 29 is moreextended than the first region 28.

Each the first regions 23, 29 of the first and second transferelectrodes 25, 30 is formed of, for example, polysilicon, while each thesecond regions 24, 29 is formed of, for example, polysilicon doped withdensity impurities, in order to increase the resistance of the lattermore than that of the former.

Referring now to FIG. 4, there is shown a charge coupled deviceaccording to a second embodiment of the present invention. As shown inthis figure, the charge coupled device is comprised of a firstconductive semiconductor substrate 40, e.g. p type silicon substrate,overlaid by an n type BCCD region 41, a second conductive charge coupledregion, covered with an insulating film 42 on which a plurality of firstregions 43 and a plurality of lower resistant second regions 44 areformed, each of the former along with nearby one of the latterconstituting a first transfer electrode 45 and on which a plurality offirst regions 48 and a plurality of lower resistant second regions 49are formed, each of the former along with nearby one of the latterconstituting a second transfer electrode 50 which is as high as thefirst transfer electrode 45, said first transfer electrode 45 beingpresent between the second transfer electrodes 50.

The first region 43 of the first transfer electrode 45 is adjacent tothe first region 48 of the second transfer electrode at an interval ofthe insulating film 46 with the second region 44 of the former 45 beingadjacent to the second region 49 of the latter 50 at that interval.

In the charge coupled device according to the second embodiment of thepresent invention, an impurity region 27 to serve as a barrier iscomprised, which is formed below each of the second transfer electrodesin the vicinity of the surface of the BCCD region 21. In the firsttransfer charge 25, the first region 23 is more extensively formed thanthe second region 24, whereas, in the second transfer charge 30, thesecond region 29 is more extended than the first region 28.

Each of the first regions 23, 29 of the first and second transferelectrodes 25, 30 is formed of, for example, polysilicon, while each ofthe second regions 24, 29 is formed of, for example, polysilicon dopedwith density impurities, in order to increase the resistance of thesecond regions more than that of the first.

With reference to FIGS. 5A and 5B, the operation of the charge coupleddevice according to the present invention will be described. While FIG.2A is an example of the clock pulses applied to the transfer electrodeof two-phase charge coupled device, FIG. 2B shows a potentialdistribution induced within a semiconductor when applying the clockpulses to the transfer electrodes, and a migration course of chargesdepending on the potential distribution.

When saw tooth waves from rectangular waves, shown in FIG. 5A, areapplied, since, at t=1, a first clock pulse (HΦ₁), as shown in FIG. 5B,is in the state of "low" and a second clock pulse (HΦ₂) is in the stateof "high", the deepest potential well comes to be formed below the firsttransfer electrode 25 applied with the second clock pulse (HΦ₂), andpulse charges are trapped in the well.

At t=2, the first clock pulse (HΦ₁) is turned from the low state intohigh while the second clock is turned from the high state to the low. Atthis time, each of the impurity-doped regions 24, 29 of the first andsecond transfer electrodes 25, 30 corresponds to such changes of theclock pulses without time delay. On the other hand, in transferring theclock pulses (HΦ₁, HΦ₂) at t=2, time delay is generated in the un-dopedregions 23, 28 by RC time constant. Such phenomena are attributed to thefact that the clock pulses (HΦ₁, HΦ₂) are changed depending on thechange of the potential with time and there exists a time change due toover-shoot after the changes of the clock pulses.

Accordingly, at t=2, owing to the impurity-doped regions free of thetime delay of the transfer electrode and the un-doped regions with thetime delay, a potential gradient is generated in a transfer channel, sothat charge transfer is achieved not only by the diffusion with heatenergy but also by the force of an electrical field. Consequently, thecharge transfer in the charge coupled device according to the presentinvention is superior in efficiency and improved in velocity.

At t=3, the clock pulses (HΦ₁, HΦ₂), as shown in FIG. 5a, are in astatic state, so that the potential of direct current comes to beapplied, depriving the transfer channel of the potential gradient. Thedisappearance of the potential gradient forms a charge well similar tothat of the conventional structure. Repetition of such clock plusestransfers the pulse charges in one direction.

Now, let us turn to methods for fabricating the charge coupled devicesaccording to the embodiments of the present invention. Referring toFIGS. 6A through 6G, there is illustrated a method for fabricating thecharge coupled device according to the first embodiment of the presentinvention.

First, as illustrated in FIG. 6A, into a p type monosilicon substrate 20n type impurities ions are implanted, to form a BCCD region 21, a chargecoupled region, over which there is then formed a first insulating film22, e.g., silicon oxide film which, in turn, is covered with a firstconductive layer, e.g. polysilicon layer. It is also illustrated in thisfigure that high density impurities are selectively doped in thepolysilicon layer 23, so as to form a plurality of spaced-apart, firstimpurity-doped regions 24 therein. As a result, in the polysiliconlayer, the first impurity-doped region and an un-doped region alternate.

Next, as illustrated in FIG. 6B, the polysilicon layer comprising thespaced-apart, first impurity regions and the un-doped regions ispatterned by use of photoetch, so as to form a plurality of firsttransfer electrodes 25 each of which consists of a smaller part of thefirst impurity-doped region and a greater part of the un-doped region.

Thereafter, as illustrated in FIG. 6C, using a plurality of the firsttransfer electrodes 25 as a mask, the first insulating film 22, asilicon oxide film, is subjected to etch.

Subsequently, as illustrated in FIG. 6D, on the resulting entire surfacecomprising the first transfer electrodes and the substrate, there iscoated a second insulating film 26, an oxide film, and then, using thefirst transfer electrodes 25 as a mask, impurity ions are implanted intothe surface of the BCCD region 21, so as to form an impurity region 27,a barrier layer.

Following this, as illustrated in FIG. 6E, over the second insulatingfilm 26, there is formed a second conductive layer 28, e.g. polysiliconlayer, which is then selectively doped with impurities, to form aplurality of spaced-apart, second impurity-doped regions each of whichis adjacent only to the side of the first impurity-doped region of thefirst transfer electrode 25, covering not the un-doped region of thefirst transfer electrode 25 but the first impurity-doped region. As aresult, in the polysilicon layer, the second impurity-doped region andan un-doped region alternate.

Next, as illustrated in FIG. 6F, the polysilicon layer comprising aplurality of the spaced-apart second-doped regions is patterned by useof photoetch in such a way to set each the second-doped region 29 onlyat the side of the first impurity-doped region 24 of the first transferelectrode 25, to form a plurality of second transfer electrodes 30consisting of a greater part of impurity-doped region and a smaller partof un-doped region, each being between the first transfer electrodes 25.

Finally, as illustrated in FIG. 6G, a third insulating film 31, e.g.oxide film, is formed over the entire surface of the resultingstructure, followed by the formation of wiring (not shown) to applyclock pulses like the conventional structure stated. At this time, thewiring is arranged in such a way to apply the clock pulses only to theimpurity-doped regions out of the first and the second transferelectrode patterns 25, 30.

Turning now to FIGS. 7A through 7G, there is illustrated a method forfabricating the charge coupled device according to second embodiment ofthe present invention.

First, as illustrated in FIG. 7A, into a p type monosilicon substrate 40n type impurities ions are implanted, to form a BCCD region 41, a chargecoupled region, over which there is then formed a first insulating film42, e.g., silicon oxide film which, in turn, is covered with a firstconductive layer, e.g. polysilicon layer.

Next, as illustrated in FIG. 7B, the polysilicon layer 43 is patternedby use of photoetch, to form a plurality of spaced-apart, first transferelectrode 43.

Subsequently, as illustrated in FIG. 7C, using a plurality of the firsttransfer electrode patterns as a mask, the first insulating film 42 isetched.

Thereafter, as illustrated in FIG. 7D, on the resulting entire surfacecomprising the first transfer electrode patterns and the substrate,there is coated a second insulating film 46, an oxide film, and then,using the first transfer electrode patterns 45 as a mask, impurity ionsare implanted into the surface of the BCCD region 21, so as to form animpurity region 47, a barrier layer.

Following this, as illustrated in FIG. 7E, over the second insulatingfilm 46, there is formed a second conductive layer 48, e.g. polysiliconlayer on which a planarization layer 52 is formed. As a result, theresulting structure is planed in surface.

Next, as illustrated in FIG. 7F, the planarization layer 52 is subjectedto etch back, and thus exposed second conductive layer 48 is alsosubjected to etch back, in order to make the second conductive layer 48be as high as the first transfer electrode pattern 43. As a result, aplurality of second transfer electrode patterns 48' are formed, existingbetween the first transfer electrode patterns 43.

Finally, in illustrated as FIG. 7G, over the resulting structure inwhich the second transfer electrode pattern 48' and the first transferelectrode pattern 43 alternate, there is formed a third insulating film51, an oxide film, and then, high density impurity ions are implantedinto a portion of each the first and the second transfer electrodepatterns 43, 48', so as to form a first impurity-doped region 44 in eachof the first transfer electrode patterns 43 as well as a secondimpurity-doped region 49 in each of the second transfer electrode 48',both of the impurity-doped regions being adjacent to each other. As aresult, both a plurality of spaced-apart first electrodes 30 each ofwhich consists of the first impurity-doped region 44 and the firstconductive layer 43 and a plurality of spaced-apart second electrodes 50each of which consists of the second impurity region 49 and the secondconductive region 48 are formed simultaneously. Although not shown inthe figure, wirings are formed in order to apply clock pulses like theconventional structure stated. At this time, the wirings are arranged insuch a way to apply the clock pulses only to the impurity-doped regions44, 49 out of the first and the second transfer electrodes 45, 50.

As described hereinbefore, the charge coupled devices according to thepresent invention, capable of utilizing the force of electrical field,are superior in charge transfer efficiency as well as charge transfervelocity to the conventional one utilizing only the diffusion with heatenergy. Therefore, they have capability to improve the performances ofhigh picture quality solid state image sensing devices as well as timedelay devices, which both necessitate a charge coupled device andoperate at high frequencies. In addition, a solid state image sensingdevice employing the charge coupled device according to the presentinvention is not degraded in a dark state of generating a few number ofpulse charges.

Whilst the present invention has been described with reference tocertain preferred embodiments, it will be appreciated by those skilledin the art that numerous variations and modifications are possiblewithout departing from the spirit or scope of the invention as broadlydescribed.

What is claimed is:
 1. A charge coupled device, comprising:a firstelectrode consisting of a first region and second region having lowerresistance than the first region; and a second electrode consisting afirst region and a second region having lower resistance than this firstregion, the first region of the first electrode being adjacent to saidfirst region of the second region at an interval of an insulating film.2. A charge coupled device according to claim 1, wherein the firstregion of the first electrode takes a greater part in the firstelectrode than the second region of the first electrode.
 3. A chargecoupled device according to claim 1, wherein the second region of thesecond electrode takes a greater part in the second electrode than thefirst region of the second electrode.
 4. A charge coupled device,comprising:a first conductive semiconductor substrate; a secondconductive charge coupled region formed on the first conductivesemiconductor substrate; a plurality of spaced-apart, first electrodesformed on an insulating film atop the second conductive charge coupledregion, each of which consists of a first region and a second regionhaving lower resistance than the first region; a plurality ofspaced-apart, second electrodes formed on the insulating film atop thesecond conductive charge coupled region, each of which consists of afirst region and a second region having lower resistance than this firstregion, and is present between the first electrodes; and an impurityformed in the vicinity of the surface of the second conductive chargecoupled region below each of the second electrodes.
 5. A charge coupleddevice according to claim 4, wherein the first region of the firstelectrode is adjacent to the first region of the second electrode at aninterval of an insulating film, and the second region of the firstelectrode is adjacent to the second region of the second electrode at aninterval of the insulating film.
 6. A charge coupled device according toclaim 4, wherein the first region of the first electrode takes a greaterpart in the first electrode than the second region of the firstelectrode.
 7. A charge coupled device according to claim 4, wherein thesecond region of the second electrode takes a greater part in the secondelectrode than the first region of the second electrode.
 8. A chargecoupled device according to claim 4, wherein the first region of thefirst electrode and the first region of the second electrode arecomprised of polysilicon.
 9. A charge coupled device according to claim4, wherein the second region of the first electrode and the secondregion of the second electrode both are comprised of polysilicon dopedwith impurities at high densities.
 10. A charge coupled device accordingto claim 4, wherein the second electrode is formed on the charge coupledregion between the first electrodes, extending over the edge portion ofthe first electrode.
 11. A charge coupled device according to claim 4,wherein the first electrode is as high as the second electrode.
 12. Acharge coupled device according to claim 4, further comprising wiringsto apply clock pulses to both the second region of the first electrodeand the second region of the second electrode.